The phenomenon where measuring signal goes in overload is caused by the high impedance of an amplifier. Whenever the input impedance is higher than for example 10 MΩ that channel will go into overload. For example: for STG amplifiers in the 10V range where the input impedance is higher than 1 GΩ even the smallest input changes such as voltage, temperature, noise,... can cause the signal to go into overload. 

A quick test to check this is:

In channel setup set the measuring range to 0.1 V and click on the "Short On" button:


All channels will go to 0,0000V or whatever the specified error of each module is.

When the 50V range is selected in DC coupling the values are OK, because the input is similar to 0V, impedance is 1 MΩ so this does not go to OVL. When the 10V measurement range is selected, (or is lower) then the impedance is 10 MΩ and the input goes to OVL. 

With selecting the AC coupling the input impedance changes, it gets lower (under 1 MΩ) so this is why the input does not go in OVL.